Radionuclides are used in various fields of technology and science, as well as for medical purposes. Usually, radionuclides are produced in research reactors or cyclotrons. However, since the number of facilities for commercial production of radionuclides is limited already and expected to decrease, it is desired to provide alternative production sites.
EP 1 667 166 A2 relates to a method of producing isotopes in a light water power reactor, whereby one or more targets within the reactor may be irradiated under a neutron flux to produce one or more isotopes. The targets may be assembled into a tubing portion of a fuel rod in one or more fuel bundles that are to be loaded in a core of the reactor at a given outage. Power operations in the reactor irradiate the fuel bundles so as to generate desired isotopes, such as one or more radioisotopes at a desired specific activity or stable isotopes at a desired concentration.
The neutron flux density in the core of a commercial nuclear reactor is measured, inter alia, by introducing solid spherical probes into instrumentation tubes passing through the reactor core. It was therefore suggested that instrumentation tubes of commercial nuclear reactors shall be used for producing radionuclides.
For example, EP 2 093 773 A2 suggests that existing instrumentation tubes conventionally used for housing neutron detectors may be used to generate radionuclides during normal operation of a commercial nuclear reactor. In particular, spherical irradiation targets are linearly pushed into and removed from the instrumentation tubes. Based on the axial neutron flux profile of the reactor core, the optimum position and exposure time of the targets in the reactor core are determined. A driving gear system is used for moving and holding the irradiation targets in the instrumentation tubes.
US 2013/0315361 A1 also relates to an apparatus and methods for producing radioisotopes in multiple instrumentation tubes of operating commercial nuclear reactors. Irradiation targets are inserted and removed from multiple instrumentation tubes and converted to radioisotopes during operation of the nuclear reactor. Positioning irradiation targets are provided to properly position other irradiation targets at desired positions within or near the nuclear core. The positioning targets can be made of an inexpensive inert material or of a magnetic material, and may be held in the instrumentation tube by means of a magnetic latch. After irradiation, the targets are delivered from the instrumentation tube into a harvesting cask, and the positioning targets may be sorted out from the harvesting cask due to their markings or physical properties.
WO 2014/107218 A2 discloses a retention assembly including a restricting structure, such as a fork, for selectively blocking the movement of irradiation targets through a pathway and into/out from instrumentation tubes. Positioning targets are provided to prop up the irradiation targets. The positioning targets may be ferromagnetic. A positioning detector is used to operate the restricting fork based on the presence of magnetic members such as the positioning targets.
CA 2 792 593 A2 describes an apparatus and methods for producing radioisotopes in instrumentation tubes of operating commercial nuclear reactors. Irradiation targets are inserted and removed from instrumentation tubes during operation and converted to radioisotopes. The irradiation targets may further include a tracking target located at a known position among all other targets that is fabricated of a material that is different from all other targets and permits tracking or locating of the irradiation targets. For example, the first and last irradiation target may be fabricated of a ferromagnetic material that can be tracked with a magnet sensor, or may be fabricated of a material converting to a different isotope product that can be detected with a radiation sensor.
Conventional spherical probes for use in a ball measuring system in the core of a commercial nuclear reactor are driven into and out from the instrumentation tubes using pressurized gas. Therefore, the irradiation targets designed for use in the instrumentation tubes must be able to withstand high mechanical loads. In addition, the targets are usually produced from parent material having high isotope purity. Preparation of the irradiation targets is therefore very expensive.
However, the neutron flux density in the core of a commercial nuclear reactor is not homogeneous and may be insufficient for converting the irradiation targets to the desired radionuclide at various axial positions of the instrumentation tubes. Generally, the neutron flux density is higher in the middle section of the core than in the areas at the top or bottom thereof. In addition, spacer elements between the fuel rods may also block the neutron flux at specific axial positions in the core. Accordingly, a selective positioning of the targets is required to avoid waste of expensive parent material due to insufficient activation.
If the expensive irradiation targets are positioned in the upper or lower part of the nuclear reactor core, the parent material will not be converted completely to the desired radionuclide. The incompletely activated irradiation targets cannot be used in industrial or medical applications and therefore will have to be sorted out and disposed or stored according to their half-life until a re-use is possible. Sorting out of the incompletely activated irradiation targets will increase safety hazards and can be done in hot cells only. Moreover, the time required for separating completely converted irradiation targets from incompletely activated targets causes a depletion of the radionuclides in the converted targets due to radioactive decay. In addition, disposal of the incompletely activated irradiation targets increases the amount of nuclear waste and thus the costs of producing the desired radionuclides.